A distance measurement device for measuring the spatial distance by using a laser light is known.
For example, as shown in FIG. 14, in a distance measuring apparatus 7, an optical modulator 72 modulates a laser light from a laser light source 71 and outputs a modulated light B01, and B01 illuminates a distance measurement object O through a half-mirror 73. The reflected light B11 is detected by a photodetector 74 and is transformed into an electric signal.
A mixer 75 combines a signal from an oscillator 76 for driving the optical modulator 72 and a signal from a local oscillator 77, and a mixer 78 combines the signal from the local oscillator 77 with the signal from the optical detector 74. The phase of the output signal from the mixer 75 is compared with the phase of the output signal from the mixer 78 by a phase comparator 79 and the distance to the distance measurement object O is measured by the phase difference of the signals.
However, since the distance measuring apparatus 7 detects the phase there is uncertainty of integer times of 2π. Therefore if the approximate distance to the distance measurement object O is unknown the distance cannot be determined. Furthermore the electric circuit for detecting phase in the distance measuring apparatus 7 is complicated and expensive.
In the measurement device of FIG. 14, only the distance between the laser light source 71 and the distance measurement object O can be measured and the laser light path needs to be changed in order to measure the distance between the laser light source 71 and other distance measurement objects. It is practically impossible to measure the distance to plural distance measurement objects simultaneously using the laser light source 71.
Therefore we have proposed a distance measurement technique for detecting the path length difference of two light paths with a high degree of accuracy by TPA (Two-Photon Absorption) using two laser light sources with different frequencies modulated by the same frequency (see IEEE Photonics Technology Letters vol. 17 No. 12 pp 2682-2684, December 2005).
By using this distance measurement technique, the path length difference of two optical paths can be detected as a sine wave cycle of the detected signal when the modulation frequency is swept.
However, the optical modulators (formed by LN substrates) and optical wavelength filters in use of these distance measurement techniques have complicated structures and are expensive.
The object of the present invention is to provide a distance measurement system with a simple configuration at a low cost for simultaneously measuring the distance between a laser light source and plural points or the distance between two points using a laser light source with a high degree of accuracy.